1. Field of the Invention
The present invention, in general relates to telescopes and, more particularly, to a variable field of view telescope that is adapted for simultaneous use with visible and infrared wavelengths.
Cassegrain types of telescopes are well known reflector-types of telescopes that are used in both the recreational arts for astronomy and ground observation and also for commercial purposes as well as for certain military applications. In general, they offer a longer focal length in a more compact package. It is desirable with all optical systems to obtain as much light energy as possible (i.e., to provide a large aperture) in as compact and rigid a structure as is possible.
Variable field of view telescopes that include a plurality of field of view optical groups for insertion into the optical path are also known. The known prior art devices utilize a “C” structure that extends in an arc from the base of a primary mirror to a position over the primary mirror. A turret attached to the upper end of the C structure is used to suspend the plurality of optical groups a predetermined distance over the primary mirror.
The plurality of optical groups that are disposed on the turret are arranged for sequential insertion of one group into the field of view following the simultaneous withdrawal of a preceding group from the field of view. Accordingly, one optical group is always disposed in the field of view.
While providing the benefit of being able to withdraw and then insert one of several optical groups into the field of view (to affect the field of view), there are several disadvantages inherent with this type of design. An important disadvantage is that the C structure incurs considerable flexing that is an inherent characteristic due to the length of the arm and the fact that it is supported only at the base of the structure.
This can cause misalignment of the optical groups and resultant distortion in the optical path. Also, because of the long length of the C structure, thermal expansion and contraction effects can further contribute to misalignment and error along the optical path or limit the temperature range in which the equipment can properly function.
Also, the C structure obstructs a significant quantity of light energy that would otherwise impede upon the primary mirror. The C structure provides only one point of mechanical support. Therefore, it must be mechanically large and strong enough so as to sufficiently limit flexing and vibration. Because the structure is supported on the outside of the primary mirror, a long moment arm is also created from the base of the C structure to the turret. It is the long moment arm that is subject to flexing and the turret that is subject to vibration.
Vibration can be in response to any mechanical energy that the C structure experiences. For example, vibration of the engine(s) that propel the vehicle or aircraft may be transmitted to the C structure. While there is always the possibility for vibration of the moment arm, there is also the possibility that the length of the arm can resonate in a harmonic frequency thereby increasing the amplitude of vibration.
To minimize flexing and vibration an amount that permits utilization of the C structure design, it is built as large and as strong as it needs to be in order to function in any given environment. However, this is not desirable because it is preferable that the C structure be as small as possible so as to minimize the amount of light energy that it obstructs. Accordingly, a tradeoff is made in the prior art design that sacrifices light gathering ability for necessary rigidity.
Also, the larger the C structure is, the heavier it also becomes. This is another limitation that is an especially important consideration in various circumstances, for example, when the telescope is used in an aircraft or spacecraft. While it is desirable that a telescope be lightweight, this is crucial in certain applications.
The prior art design also relies upon the use of moveable hard stops to repeat the position that the plurality of optical groups are maintained in. This limits the speed by which optical groups can be changed without causing damage.
Furthermore, when the plurality of optical groups are moved into and out of the optical path, it is desirable that precise repeatability of their position occur, else the optical path is adversely affected. The prior art designs that require variability in the stops are unable to provide optimum repeatability in the positioning of the optical groups.
An especially significant limitation with the prior art designs is that the field of view changes in steps according to the number and optical characteristics of the various optical groups. It is desirable to be able to step in IR and also to provide a progressive zoom capability in visible as well, which the prior art fails to provide.
The prior art designs are also limited in their ability to provide both IR (infrared) and visible light gathering ability. Separate optical paths are typically required. If the telescope is used in a gimbal ball, for example, this would result in having two apertures in the gimbal ball, one for visible and one for IR. This is undesirable for many reasons.
There are many light frequencies that may be of interest. It is desirable to be able to view two or more channels simultaneously and also to change the field of view (FOV) from between wide angle FOV (lower magnification) and narrow FOV (higher magnification) capabilities. Prior art designs have been unable to effectively switch back and forth between infrared and visible light while also providing a zoom capability.
For many applications it is desirable to be able to zoom in the visible spectrum. For example, if after detecting an object of interest using a wide field of view (in the visible spectrum or IR spectrum), it is desirable to be able to narrow the field of view and zoom in to obtain a closer examination of the object. If the object of interest was first discovered as a presence in the infrared spectrum, it may be desirable to switch study to the visible spectrum and zoom in accordingly.
Prior art designs have provided limited zoom capability in the visible spectrum. This is because a rigid and relatively long physical path is required to house the optical elements that typically accompany an optical zoom system. A recording camera or other type of transducer may be placed in the field of view, as desired.
Furthermore, space is often a commodity in short supply, especially if the telescope is to be housed in a gimbal ball.
Accordingly, it is desirable to be able to provide a space to accommodate the optical elements of a visible zoom optical system and do so in such manner as to provide an optimally long linear path that is especially rigid. It would also be a significant and unexpected benefit if the structures used for the visible optical zoom system were able to provide increased rigidity to the overall structure for supporting an optical telescope.
As mentioned briefly hereinabove, there are many possible frequencies of light (electromagnetic) energy that may of interest depending upon the application. It is desirable to provide versatility in such an instrument to change the transducers and analyze the relevant spectrums that may be of interest.
Another prior art limitation concerns the mounting of the gyroscope that is used to determine position of the primary mirror. As the primary mirror is used in long focal length (i.e., high magnification) applications, its positioning is of critical concern.
Prior art designs have included the gyroscope inside of the gimbal ball, for example, but the gyroscope has been positioned distally away from the primary mirror. A number of support structures exist in prior art designs that are disposed intermediate the primary mirror and the gyroscope. Tolerances, vibration, expansion and contraction, all combine to introduce a potential for error occurring between the actual position of the primary mirror and where the gyroscope “thinks” it is pointing.
It is desirable to limit such error to an absolute minimum. Positioning data may be obtained from GPS (global positioning satellite) data. An IMU or an inertial measurement unit includes a combination of three rate instruments and three accelerometers. The preferred type of rate instruments include fiber optic gyroscopes (FOG). Typically, IMUs are used to calculate changes in direction as well as position information by measuring outputs from the FOGs and accelerometers. Accordingly, motion in all directions is accounted for. When GPS data is combined with IMU output and computer circuits are utilized (to provide a corrective signal), and Inertial Navigation System (INS) is provided.
Accordingly, the ability of the INS to correct for changes in position (for example, of the aircraft) and to maintain the primary mirror pointing precisely on an object of interest, especially at long focal lengths, is dependent upon how accurately the direction in which the primary mirror is pointing is known.
If the distance between the primary mirror and the IMU introduces a variable then the IMU's output is subject because it indicates changes in the IMU's position which do not correlate one to one with changes in the primary mirror's position.
Accordingly, it is especially desirable to reduce error between the output of the IMU and the actual direction the primary mirror is oriented.
Accordingly, there exists today a need for a structure for supporting an optical telescope that overcomes these prior art limitations.
2. Description of Prior Art
Telescopes and zoom lenses are, in general, known. For example, the following patent describes various types of these devices:
U.S. Pat. No. 5,907,433 to Voigt et al, May 25, 1999; and
U.S. Pat. No. 5,940,222 to Sinclair et al, Aug. 17, 1999.
While the structural arrangements of the above described devices, at first appearance, have similarities with the present invention, they differ in material respects. These differences, which will be described in more detail hereinafter, are essential for the effective use of the invention and which admit of the advantages that are not available with the prior devices.